#include <bits/stdc++.h>
using namespace std;
bool comp(const int &a , const int&b)
{
return ((a > 0 and b > 0) ? true : ((a == 0) ? false : true));
}
int32_t main()
{
vector <int> a = {1 , 0 , 0 , 3 , 12};
sort(a.begin() , a.end() , comp);
for(int &x : a)
cout << x << ", " ;
cout << endl;
return 0;
}
The aim is to move all the zeroes to the back keeping the relative order of all other non-zero elements intact. I thought the comparator would do the job as it changes the relative order of two elements a and b if a is equal to 0 and not in any other case, and print the final output as :
0, 0, 1, 3, 12,
However, it printed this :
12, 3, 1, 0, 0,
Why is the comparator failing?
THIS REALLY HELPED : Custom Comparators
The comparator function in stl:sort requires strict weak ordering. Your comparator causes UB while sorting. Read more here — Geothermal's comment.
Looks like you need std::stable_partition
read this blog https://codeforces.net/blog/entry/70237
Thanks, that blog was not of much use but I found a link to a good resource in the comments which certainly helped. :).
Find it here.
Auto comment: topic has been updated by wannbegood (previous revision, new revision, compare).
There is no need for a custom comparator. The Standard Library already has a function to move all elements that are not equal to a certain value to the front: std::remove. However, it does not necessarily move the "bad" values to the end. So the end of the range is in an undefined state. To overcome this, you can use std::fill. Here is an example.